Process for the carburization of ferrous metals



Patented Sept. 25, 1951 mtooriss FOR E CARBURIZATioN F FERR'OUS METALS PaulZM; Leiningen Grand Island, and Hugh FarreILZ'Niagara Falls; N. Y.,,-assignorsto I Punt dfigNBlllQlll'gigKc Company, ,Wilmington,

UN Iran: STATES PAT ENTJ OFFICE Del.,a corporation of Delaware No Drawing:

ing.

rous metal articles."

mation.

of: molten bath ingredients;

Api lication 1 ailllary I Serial No. 3,8

3 Claims} (Cl. 14815.5')

The objectsof thisinvention can be achieved by preparing] a fused saltbath composed essentially of alkali-metal carbonates and chlorides,

and finely divided; carbon, and passing through the bath an oxygen containing gas taken from: the group consisting of carbon monoxide and.

gases capable of reacting with carbon to form carbon-monoxide in thefuse'd salt bath.

Ferrous structures or articles, i. e., structures or articles composed of iron,- ;steel, and-iron'andof thecarbonate:

inert alkali or alkaline earth metal salts, such as-alkalimetal or -e:lkaline. earth metal chlorides steel alloys, when immersed in the above-deor alkalilmetal =orlalkaline earth metal carbonscribed bath at a temp ur Of 15500 0 ates, and a carburizin'g: material in the form of F.- for a period of fifteen-minutesorlonger will a cyanide; Inxthese bathsthe cyanideis debe actively carburized; I p I a a v composedand the earbonu nd nitrogenof the Thecarburizing bath of the present'inventionsame penetratesinto' 'the suniace of the objects: is preferably prepared as v follows: to be.hardened-:or cai liurizedi- The addition-oi A mixture of alkali. metal.-(sodiumor potas-' carbon to a cyanide case hardening bath=desium) carbonate and-alkalirnetal-(sodium' or pocreases-itscarbuiizing aetion-i =-Cyanide-containtassiuml chloride,- preferably not necessarily, ing 1'usedbatlisare, however, objctionable containing to 75% view of therelatively -highcost-"of-tlie-cyan-ide and 25% to 75%- of the chloride ismeltedand with 'wh-ichthebath must-be -constantly i'eplen='--- held at a temperature of about'1500 F. to 1700-F. ishedand the toxic properties of cyariides': for several hours: Finely divided carbon is added Cyanide-free, fused salt, carburizaitlon-baths to the fused carbonate-chloride bath in an have also been usedhe'ret'ofore, for example, a 25 amount'in excess of' tha-t' which'willremain disfused, inert, salt bath containing silicon carbide. persed in the bath, and isthoroughly mixed into Baths of this. latter -typeahave been foundqobthe bath. Astream of a gas taken" from the jectionablein that not. only do they produce very v group consisting of carbon monoxide and gases poor carburization but the silicon 'carbidemust' capable of reactingwith carbonitoform carbon be constantly replenished and the metal silicates monoxide, for example; carbon-dioxide,- oxygen; formed in such bathscause objectionable sludgenitrous oxide; nitric oxide; steamyair; or'mixtures thereof, is bubbled throughuthe carbonecontain Itis an object :of this invention to movidej-an ing, fused bath ata rateobetween'50 and 5001cc; improved-process for" the. carburization of fer- N. P. T;, i. e., measured at 0? C; and'lGQmma'I-Ig, per 100-poundsof theabath; The carburizing It is another-object of this invention to provide; activity of the. bath increases gradually; for a a fused, carburi'zation: bath that is. capable of: period of' twenty to. twenty-four: hours, after producing a nitrogen-free, carburized case: which it'llev'els 'ofixahdrremainsssubstantially con.- It is still another object of this invention to stant; I provide a cyanide-free, fused, carburization bath 40 Thisdarburizingjbath is operative t ej m Which-is llflliisulliecliT objeetio ble sludgerior enlfer'rous structures withinia temperature range of 1550 F. to 1750''F. when=immersedin the bath; It is y no er bj t r his invention o The fused. carburizingsbath, as-above stated; provide a cyanide-freetused,carburization bath preferably"shou1d.1-contain;25% to 75%] of the which-is not subject-:to trequent replenishment alkali metal carbonate1and .25:% to 75% of the V alkali metal chloride to obtain the best carburiz It is a still further object of: thiSiIlVBiltiOIltOi ingiconditions. 'Ihese-prdportions arenotyhdW- providea cyanide-thee, iused, carburizingr bath ever, essential since. carburizationiwillttake place that-is non-corrosive'iand washesfreely from the in the absence oftone-orthe other:substance: For carburized articlesaand whichis capableofpro- 5o optimum carburizing conditions; it isla'lso deducing an" activet'carburizationz at: a compara-l sired that' at 1easti25%' of. theimixtu're be com: tively: low temperature;- :between: 1550 F. and posed-of potassifim-salti The' presence'of atl'east 25% of potassium salt appears to induce more Other objects of the invention will appearliere active carburization: than w1iere the entire amount 01' the 'salt' is -coxiipos'ed of s'odiiirr'i salts'.

Minor amounts, i. e., less than 25% of the weight of the inert salts in the bath, of other salts may be present without deleterious effect. For example, calcium, barium or strontium carbonates and chloridesyina; total amount of less than 25 will have'no harmful effect. Alkali metal or alkaline earth metal cyanides may be present in the bath, it .being understood, howeventhat J the presence of cyanides will cause nitriding along with the carburizing action. i

The carbon employed should be finely divided,

a be maintained fused at a fairly constantly conpreferably 50 mesh or finer, so that it maybe rapidly and uniformly dispersed throughout the; melt. All forms of carbon are operative topro-- duce useful carburization, preferably, however,

finely divided graphite is employed because of the greater carburizing activity obtained with this material.

trolled temperature, and means be provided for passing thegas'into'rthe molten bath at the desired rate and fairly uniformly dispersedacross the cross-sectional area of the pot. Preferably, the .pot and bubbling means immersed in the pot are constructed of corrosion-resistant materials,

for-'exampla-nick'le, iron, chromium-nickel alloys, or compressed graphite.

Although we do not wish to be restricted by any theories underlying the principles of the The amount of carbon mixed with the fused bath should be controlled within certain rather broad limits. It ispreferably employed in an amount in excess of that amount which will remain dispersed throughout the bath, so that a certain excess will float on the surface of the bath. If too large'an excess is employed, difliculty 'will be encountered from sludge formation; if toolittle is employed, maximum carburizing activity will not be obtained. The amount of carbon employed is dependent upon the temperature and composition of the fused bath. Depending upon these conditions, between 0.1% and 10% carbon may be usefully employed. For example in a 50% KCl-50% K2003 base melt having a temperature of 1700" F., the optimum amount of carbon employed should be between 0.5% and 10% by weight of the base melt. A 50% Na'zCOs-50% 'KCl base melt having a temperature of -1600 F: will'have optimum carburiz-' ing action if the carbon constitutes between 0.1

and 5.0% carbon. f v

Carbon monoxide "or any oxygen-containing gas which is capableof reacting with carbon to form carbon monoxide at the above-mentioned temperature range-may be-used in the carburization with the above-described bath. The rate of gas flow to be employed will depend upon the kind of gas used, the composition of the base melt, the typeof gas bubbling apparatus and the dimensions of the pot used, as well as the operating temperature of the bath. Generally, the

carburizing activity of the bath will be found to increase with an increase in the rate of gas flow up to a definite maximum .beyond which the carburizing activity falls ofi slowly. For example, when employing a basemelt composedv of pounds Na-2CO3 and .50 pounds KCl at a temperature of 1600 F. in a 12-inch diameter pot and a bubbler having uniformly dispersed holes over the bottom of the pot, the optimum rate of flow for carbon dioxide gas was found tobe between 50 and 500 cc. per minute at normal conditions of temperature and pressure. Good carburization may, however, be obtained by use of gas flow as low as 25 cc. perminute, orby use of gas flow as high as 2000 cc. per minute, N. P. T., per 100- pounds of bath.

If simultaneous carburizing and nitriding is pre'sentinvention, it is believed that in the carburization of ferrous metals it is essential that two conditions be present: i first, the presence 1 of a supply of 1 carbon monoxide and second, a

means of eliminating carbon dioxideformed durwhereby an exceedingly efllcient and active .car-v burization is produced :without objectionable sludge formation, very infrequent replenishment of carbon, and under comparatively low, tempera.-.-

tures.; r

The follo-wing examples are given to illustrate certain .p ieferredmethods of carburization of ferrous metalsin accordance with this invention, it being understoodthat these. examples are to be taken solely as illustrative and; not limitative of theinvention. f v

. x e I I Fifty (50) I pounds of NaaCOs and 50 pounds of KCl were charged into a 35% Ni-15% Cr heatresistingalloy pot 12 inches in diameter and 24 inchesdeep. The salts were-melted and the temperature raised to 1600 F; -The meltwas held at this temperature overnight; Powdered graphite of 200 mesh and containing about 98% carbon and a maximum of 2% ash was then added to. the bath and :mixed therein; An excess was employed so that a layer of graphite-remained floating on the top of the bath. The excess, however, was insufficient to produce sludge formation in themelt. x

The bubbler was made of 1"nickel tubing'of welded ..construction,. having three horizontal equi-length legs (5") atangles of with each other. Each leg was perforated on each side with six holes (35%" diam). A

Carbon dioxide gasgwas, started through the bubbler, which had been preheated toremove all traces of moisture. The .-rate of flow was then adjusted to 200 cc.=-per.=minute N. .-P. T. and the pressure head was setat about 6 inches of mercury above. atmospheric, pressure. The bubbler After operating this fashion overnight, an

j Per Cent Increase, out Per cent a:

analyzing for carbon, subtracting the carbon found from the original or core carbon content, summing these carbon increments over the whole bar, and multiplying by 100. V

For the purpose of comparison, a similar S. A. E. X1020 steel at1600 F. in the standard NaCN case hardening bath. This showed only 89- carbon units. In addition, 78 nitrogen units were also obtained, since cyanide activated baths both carburize and nitride. Analysis of the first cut from the bar treated. in the COz-f-C activatedflbath showed. noincreasse in nitrogen content.

An S. A. E. X1020 test pin that was water quenched from the above'gas activated bath after one hour treatment at 1600-showed a Rockwell C hardness of 55'. Case depth to 0.30% C on the test bar was-0.0-17"" as revealed' in the above gradient.

Test gears oil quenched from the above bath washed much more readily than similar ones quenched from an alkaline earth activated type cyanide bath.

Example II p:m=0.0096t"- where pan=case depth in inches to 0.30% C, and

t=ti1ne of treatment in hours.

This same bath was then raised to 1700 F. with the same rate of CO2 flow. Three more test bars were treated for one, four, and sixteen hours, respectively, at this elevated temperature. Analyses indicated that carburization at 1700 F. obeyed the following expression:

p31i=0.019t- Example III This example illustrates the use or carbon monoxide gas.

A charge of 50 pounds of NazCOa and 50 pounds of KCl was melted as described in Example I. Then, in the presence of excess finely-powdered test bar was treated for one hour crcpmt sa stream 'of CO was bubbled through themelt at therateof300cc. per'minutefNrP. T. carer S.- A. 'E'.- Xl020 steel =containing i-0.22% carbon was treated for-one Murat i600 F.,"and was analyzed for carbon "as previously described. The following results were :obtained r'r Cent" Increase out 0 Per. Oentz'O 0. 54 0. 32 S1A- .E, K1 0200110 0.47 0.25 hour, 1 600, F. 0-34 0.12 0221* 0.05;. 0.23 0.01 0.22 (core) 0.00

. Example-JV I I This example illustrates "the of nitrous oxide asactivatiriggas. v

A 50% KGl'50% Na2CO5 bath"-wa's startedas described i'nfExample I, but was activated with a stream of-NzO (instead of CO2) at" the rate or 50 cc. per minute N. P. T. A sample test bar of SIA. E. X1020 steel, treated at '1 600'F. for-one hour under" these conditions, showed carbon pick-up of 51' Ercmple" V example illustrates the effect of added NaCN upon the case hardening action of'thi's type of bath. Forty-four (44) pounds of NazCOs+44 pounds of KCl .weremelte'd as in Example I. A heavy layer of finely-powdered graphite was'add'ed'and 200cc. vper minute N. RT. of Coifwa'slpassed through the melt using the nickel bubbler." The next morning a test bar of S. A. E. X1020 steel was treated therein for one hour at 1600 F.

Five (5) pounds of 96% NaCN were next added to the melt. Test bars were treated for one hour at 1600 F., one, five, and twenty-two hours after the cyanide addition. Carbon dioxide was bubbled throngh the melt throughout the entire experiment.

Gradient cuts were removed from the test bars as described in Example I. These cuts were then analyzed for carbon and nitrogen. Following are the results:

Two effects are apparent: First, the addition of cyanide causes nitriding; second, immediately following the cyanide addition, carburizing decreases slightly, then picks up toa value higher than without the cyanide present.

One advantage of the process of the present invention is that it permits one to carburize in a molten media free of cyanides and cyanogen compounds that are toxic, expensive, and cause simultaneous nitriding, which is often objectionable. A second advantage of the process is that it permits one to case carburize in a bath which does not contain compounds which make subsequent washing of the treated parts diificult. A third advantage of the process is that it produces consistently good carburizing with no salt replenishment required. except to mm: mm.

. .dragout onathe work.

.This eliminates =-.entirely the-necessity for bailing outa portion of the bath daily to make room for the replenishing agent; as .mustxbe done in the case ,of most-other molten .case hardening. processes. '-A- fourth advantage of the process is the elimination of ob ectionable thickening and sludge formation in the melt. Another advantage'of the process is its economy. Still another-advantage is the fact that this process enables one to case carburize with the above advantages in the lower portionof the normal case hardening temperature range, where direct quenching following carburizing is practi-. cal.

Throughout-the specification and claims, any reference to parts, proportions and percentages refers to parts, proportions and percentages by weight unless otherwise specified.

Since it is obvious that many changes and modifications .canbe made in the above described details without departingfrom the;nature. and spirit of the-invention, it is to beunderstood that the; invention is not to be limited tosaiddetails except as set-forth in the appended claims.

What is claimed is: 1. @The process of carb'urizing ferrous [metal structures which comprises immersing said structures in a fused salt bath having an active carburizing action on ferrous metals, the carburizing action of which is produced by incorporating finely-divided. carbon in a fused salt bath having a temperature between 1550 F. and 1750 F; and consisting essentially of at least one carbonate selected from the class consisting of sodium carbonate and potassium carbonate and at least one ,chloride selected from 'dium chloride and potassium chloride, and pass- "ing into said batha gas selected fromthe class the class consisting of so consisting of carbon monoxide and an inorganic oxygen-containing gas whichreacts with carbon to. form carbon monoxide in said fused bath.

2. The process of carburizing ferrous metal structures as defined in claim 1 in which the fusedsalt' bath contains between 25% and 75% by weight of one of said carbonates and between 25% and 15% by weight of one of said chlorides.

3. The process of carburizing ferrous metal structures as defined in fused salt bath contains between 25% byweight'of one of said'carbonates and between 25% and 15% by weight of one salts. .PAUL M. LEININGER.

HUGH FARRELL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES Materials and Methods, pp. 75-77, July 1947.

- Transactions, published by American Society for Metals, vol. 20, 1932, pp. 347- 352.

claim 1 in which the and '75%- of said chlorides. at least 25% of said salt consisting of potassiurrf 

1. THE PROCESS OF CARBURIZING FERROUS METAL STRUCTURES WHICH COMPRISES IMMERSING SAID STRUCTURES IN A FUSED SALT BATH HAVING AN ACITIVE CARBURIZING ACTION ON FERROUS METALS, THE CARBURIZING ACTION OF WHICH IS PRODUCED BY INRORPORATING FINELY-DIVIDED CARBON IN A FUSED SALT HAVING A TEMPERATURE BETWEEN 1550* F. AND 1750* AND CONSISTING ESSENTIALLY OF AT LEAST ONE CARBONATE SELECTED FROM THE CLASS CONSISTING OF SOBONATE AND POTASSIUM CARBONATE AND AT LEAST ONE CHLORIDE SELECTED FROM THE CLASS COSISTING OF SODIUM CHLORIDE AND POTASSIUM CHLORIDE, AND PASSING INTO SAID BATH A GAS SELECTED FROM THE CLASS CONSISTING OF CARBON MONOXIDE AND IN INORGANIC OXYGEN-CONTAINING GAS WHICH REACTS WITH CARBON TO FORM CARBON MONOXIDE IN SAID FUSED BATH. 